Broad Climate Change Question

I've certainly hear and read tons of stuff that is not only less than top notch science, but also seems to answer the wrong questions.

What I am specifically interested in are scientific cases presented - not for how much damage we are experiencing or will experience - but what good it would do to, for example, reduce carbon emissions. Clearly there are problems with making absolute forecasts, but comparative ones would do as well.

For example, is there any research that would project how much less sea level rise there would be in probabilistic terms decade by decade over the next century or two if we reduce CO2 emissions by 50%?

For example, is there any research that would project how much less sea level rise there would be in probabilistic terms decade by decade over the next century or two if we reduce CO2 emissions by 50%?

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Reducing CO2 emissions by 50%? Over what time frame? A decade? Three decades? Seventy years? Longer?

A decade is not realistic. Humanity is not going to make that drastic a cut over a short span of time. Populations are still increasing, so keeping emissions per person constant still represents a good sized increase. The developing world is developing, exacerbating the problem. Nobody has studied reducing emissions by 50% within a decade because that's a pure sci-fi scenario.

There have been lots of different scenarios that have been analyzed in the scientific media. Too many, in fact. The next IPCC report will cut the huge number of scenarios down to four. These are the Representative Concentration Pathways (RCPs), identified by their peak forcing between now and the end of the century.

The most aggressive scenario investigated for the upcoming IPCC report is RCP-3PD. The "3" represents a peak forcing of about 3 watts/m2. This scenario entails severe restrictions greenhouse gas emission. Even with that, emissions increase over the next few years, drop to 2010 levels in 2020, and are finally halved from 2010 levels in 2040. This scenario is going to be a very hard sell.

The next most aggressive is RCP-4.5, which has a peak forcing of about 4.5 watts/m2. This scenario isn't near as harsh as RCP-3PD. Emissions peak in 2040 at about 30% above 2010 levels and then drop, reaching half the 2010 levels in 2080. There next two scenarios are RCP-6 and RCP-8.5. RCP-6 represents a half-hearted effort to control emissions starting in mid-century. RCP-8.5 is emissions gone wild: Business as usual in a still-growing world.

So what are the impacts of these scenarios? That depends very much on the sensitivity of the climate to greenhouse gases and on the ability of the environment to absorb excess greenhouse gases. Sensitivity has been and continues to be the big unknown in climate change research. Just looking at CO2 alone, a doubling in CO2 content will increase the global mean temperature by 1.8 °C. There are feedbacks to consider. What are the side effects of this increased temperature?

The key problem is humidity and clouds. Water vapor is the by far the most significant greenhouse gas. An increase in temperatures increases evaporation rates, so temperature increases induced by CO2 (or any other well-mixed greenhouse gas) result in a positive feedback in humidity. That increased humidity also means more clouds. Clouds increase albedo significantly, thereby reducing insulation. This is a negative feedback.

So what's the net result? The scientific community has not made much progress in this regard. Back in 1979, the consensus was that the net forcing resulting from a doubling of CO2 levels was somewhere between 1.5 °C (representing a slight negative feedback) and 6 °C (a somewhat strong positive feedback). Climatologists didn't think the negative feedbacks were likely, moving the consensus to somewhere between 2 and 6 °C (a slight to somewhat strong positive feedback). Recently there have been some doubts cast on outright elimination of the negative feedbacks. We're right back where we were in 1979, somewhere between 1.5 °C and 6 °C for a doubling of CO2.

Even the most aggressive of these four scenarios has a near doubling of atmospheric CO2 by the end of the century. Depending on which path we follow and the sensitivity of the climate, we're looking at a 1 °C increase in global temperatures for scenario RCP-3PD and a minimal sensitivity to an increase of over 10 °C for scenario RCP-8.5 and a large sensitivity.

It's very, very late. I'll add more references to this later. Here's a start: